How PHP 8 JIT Works and What `opcache.jit=1205` Actually Means

A few points are worth clarifying before touching the configuration.

Opcache already performs optimization at the opcode level. For example, two opcodes may be folded into one.
In PHP 8, JIT is provided inside Opcache rather than as a separate subsystem.
JIT runs on top of Opcache’s existing optimizations, then uses runtime information to optimize further and emit machine code directly.
That means JIT does not replace the original Opcache optimizer; it extends it.
At the moment, PHP 8 JIT only supports x86 CPUs.

JIT also reuses a lot of the infrastructure already built for Opcache optimization, including data flow graphs, call graphs, and SSA.

Adding JIT settings to `php.ini`

After PHP is installed, the existing Opcache settings are not enough if you want to enable JIT. Add these options to php.ini:

opcache.jit=1205
opcache.jit_buffer_size=128M

The opcache.jit value looks cryptic at first, but it is really made of four independent digits. Read from left to right, they mean the following.

1) Whether to use AVX instructions when generating machine code

This depends on CPU support.

0: do not use AVX
1: use AVX

2) Register allocation strategy

0: no register allocation
1: local allocation at block scope
2: global allocation at function scope

3) When JIT should be triggered

0: JIT as soon as the PHP script is loaded
1: JIT the first time a function is executed
2: after one run, JIT the top percentage of most-called functions, where the percentage is controlled by opcache.prof_threshold * 100
3: JIT a function or method after it has executed more than N times, with N related to opcache.jit_hot_func
4: JIT only functions marked with @jit in their comments
5: JIT after a trace has executed more than N times, with N related to settings such as opcache.jit_hot_loop and opcache.jit_hot_return

4) JIT optimization level

Higher numbers mean more aggressive optimization.

0: disable JIT
1: JIT only branch/jump parts between oplines
2: inline opcode handler calls
3: function-level JIT based on type inference
4: function-level JIT based on type inference and the call graph
5: script-level JIT based on type inference and the call graph

What can be inferred from these settings

A few practical conclusions follow from the meaning of those four digits.

A 12x5 pattern is generally the most sensible choice if you are aiming for the best result.
For the third digit x, script-style workloads are better off with 0, while long-running web-service workloads may choose 3 or 5 depending on benchmark results.
The @jit form may eventually become <<jit>> now that attributes exist.

Benchmarking with `Zend/bench.php`

To see the difference between running with JIT disabled and enabled, the benchmark used here is Zend/bench.php.

<?php
if (function_exists("date_default_timezone_set")) {
    date_default_timezone_set("UTC");
}

function simple() {
  $a = 0;
  for ($i = 0; $i < 1000000; $i++)
    $a++;

  $thisisanotherlongname = 0;
  for ($thisisalongname = 0; $thisisalongname < 1000000; $thisisalongname++)
    $thisisanotherlongname++;
}

/****/

function simplecall() {
  for ($i = 0; $i < 1000000; $i++)
    strlen("hallo");
}

/****/

function hallo($a) {
}

function simpleucall() {
  for ($i = 0; $i < 1000000; $i++)
    hallo("hallo");
}

/****/

function simpleudcall() {
  for ($i = 0; $i < 1000000; $i++)
    hallo2("hallo");
}

function hallo2($a) {
}

/****/

function mandel() {
  $w1=50;
  $h1=150;
  $recen=-.45;
  $imcen=0.0;
  $r=0.7;
  $s=0;  $rec=0;  $imc=0;  $re=0;  $im=0;  $re2=0;  $im2=0;
  $x=0;  $y=0;  $w2=0;  $h2=0;  $color=0;
  $s=2*$r/$w1;
  $w2=40;
  $h2=12;
  for ($y=0 ; $y<=$w1; $y=$y+1) {
    $imc=$s*($y-$h2)+$imcen;
    for ($x=0 ; $x<=$h1; $x=$x+1) {
      $rec=$s*($x-$w2)+$recen;
      $re=$rec;
      $im=$imc;
      $color=1000;
      $re2=$re*$re;
      $im2=$im*$im;
      while( ((($re2+$im2)<1000000) && $color>0)) {
        $im=$re*$im*2+$imc;
        $re=$re2-$im2+$rec;
        $re2=$re*$re;
        $im2=$im*$im;
        $color=$color-1;
      }
      if ( $color==0 ) {
        print "_";
      } else {
        print "#";
      }
    }
    print "<br>";
    flush();
  }
}

/****/

function mandel2() {
  $b = " .:,;!/>)|&IH%*#";
  //float r, i, z, Z, t, c, C;
  for ($y=30; printf("\n"), $C = $y*0.1 - 1.5, $y--;){
    for ($x=0; $c = $x*0.04 - 2, $z=0, $Z=0, $x++ < 75;){
      for ($r=$c, $i=$C, $k=0; $t = $z*$z - $Z*$Z + $r, $Z = 2*$z*$Z + $i, $z=$t, $k<5000; $k++)
        if ($z*$z + $Z*$Z > 500000) break;
      echo $b[$k%16];
    }
  }
}

/****/

function Ack($m, $n){
  if($m == 0) return $n+1;
  if($n == 0) return Ack($m-1, 1);
  return Ack($m - 1, Ack($m, ($n - 1)));
}

function ackermann($n) {
  $r = Ack(3,$n);
  print "Ack(3,$n): $r\n";
}

/****/

function ary($n) {
  for ($i=0; $i<$n; $i++) {
    $X[$i] = $i;
  }
  for ($i=$n-1; $i>=0; $i--) {
    $Y[$i] = $X[$i];
  }
  $last = $n-1;
  print "$Y[$last]\n";
}

/****/

function ary2($n) {
  for ($i=0; $i<$n;) {
    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;

    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;
    $X[$i] = $i; ++$i;
  }
  for ($i=$n-1; $i>=0;) {
    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;

    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;
    $Y[$i] = $X[$i]; --$i;
  }
  $last = $n-1;
  print "$Y[$last]\n";
}

/****/

function ary3($n) {
  for ($i=0; $i<$n; $i++) {
    $X[$i] = $i + 1;
    $Y[$i] = 0;
  }
  for ($k=0; $k<1000; $k++) {
    for ($i=$n-1; $i>=0; $i--) {
      $Y[$i] += $X[$i];
    }
  }
  $last = $n-1;
  print "$Y[0] $Y[$last]\n";
}

/****/

function fibo_r($n){
    return(($n < 2) ? 1 : fibo_r($n - 2) + fibo_r($n - 1));
}

function fibo($n) {
  $r = fibo_r($n);
  print "$r\n";
}

/****/

function hash1($n) {
  for ($i = 1; $i <= $n; $i++) {
    $X[dechex($i)] = $i;
  }
  $c = 0;
  for ($i = $n; $i > 0; $i--) {
    if ($X[dechex($i)]) { $c++; }
  }
  print "$c\n";
}

/****/

function hash2($n) {
  for ($i = 0; $i < $n; $i++) {
    $hash1["foo_$i"] = $i;
    $hash2["foo_$i"] = 0;
  }
  for ($i = $n; $i > 0; $i--) {
    foreach($hash1 as $key => $value) $hash2[$key] += $value;
  }
  $first = "foo_0";
  $last  = "foo_".($n-1);
  print "$hash1[$first] $hash1[$last] $hash2[$first] $hash2[$last]\n";
}

/****/

function gen_random ($n) {
    global $LAST;
    return( ($n * ($LAST = ($LAST * IA + IC) % IM)) / IM );
}

function heapsort_r($n, &$ra) {
    $l = ($n >> 1) + 1;
    $ir = $n;

    while (1) {
    if ($l > 1) {
        $rra = $ra[--$l];
    } else {
        $rra = $ra[$ir];
        $ra[$ir] = $ra[1];
        if (--$ir == 1) {
        $ra[1] = $rra;
        return;
        }
    }
    $i = $l;
    $j = $l << 1;
    while ($j <= $ir) {
        if (($j < $ir) && ($ra[$j] < $ra[$j+1])) {
        $j++;
        }
        if ($rra < $ra[$j]) {
        $ra[$i] = $ra[$j];
        $j += ($i = $j);
        } else {
        $j = $ir + 1;
        }
    }
    $ra[$i] = $rra;
    }
}

function heapsort($N) {
  global $LAST;

  define("IM", 139968);
  define("IA", 3877);
  define("IC", 29573);

  $LAST = 42;
  for ($i=1; $i<=$N; $i++) {
    $ary[$i] = gen_random(1);
  }
  heapsort_r($N, $ary);
  printf("%.10f\n", $ary[$N]);
}

/****/

function mkmatrix ($rows, $cols) {
    $count = 1;
    $mx = array();
    for ($i=0; $i<$rows; $i++) {
    for ($j=0; $j<$cols; $j++) {
        $mx[$i][$j] = $count++;
    }
    }
    return($mx);
}

function mmult ($rows, $cols, $m1, $m2) {
    $m3 = array();
    for ($i=0; $i<$rows; $i++) {
    for ($j=0; $j<$cols; $j++) {
        $x = 0;
        for ($k=0; $k<$cols; $k++) {
        $x += $m1[$i][$k] * $m2[$k][$j];
        }
        $m3[$i][$j] = $x;
    }
    }
    return($m3);
}

function matrix($n) {
  $SIZE = 30;
  $m1 = mkmatrix($SIZE, $SIZE);
  $m2 = mkmatrix($SIZE, $SIZE);
  while ($n--) {
    $mm = mmult($SIZE, $SIZE, $m1, $m2);
  }
  print "{$mm[0][0]} {$mm[2][3]} {$mm[3][2]} {$mm[4][4]}\n";
}

/****/

function nestedloop($n) {
  $x = 0;
  for ($a=0; $a<$n; $a++)
    for ($b=0; $b<$n; $b++)
      for ($c=0; $c<$n; $c++)
        for ($d=0; $d<$n; $d++)
          for ($e=0; $e<$n; $e++)
            for ($f=0; $f<$n; $f++)
             $x++;
  print "$x\n";
}

/****/

function sieve($n) {
  $count = 0;
  while ($n-- > 0) {
    $count = 0;
    $flags = range (0,8192);
    for ($i=2; $i<8193; $i++) {
      if ($flags[$i] > 0) {
        for ($k=$i+$i; $k <= 8192; $k+=$i) {
          $flags[$k] = 0;
        }
        $count++;
      }
    }
  }
  print "Count: $count\n";
}

/****/

function strcat($n) {
  $str = "";
  while ($n-- > 0) {
    $str .= "hello\n";
  }
  $len = strlen($str);
  print "$len\n";
}

/*****/

function gethrtime()
{
  $hrtime = hrtime();
  return (($hrtime[0]*1000000000 + $hrtime[1]) / 1000000000);
}

function start_test()
{
    ob_start();
  return gethrtime();
}

function end_test($start, $name)
{
  global $total;
  $end = gethrtime();
  ob_end_clean();
  $total += $end-$start;
  $num = number_format($end-$start,3);
  $pad = str_repeat(" ", 24-strlen($name)-strlen($num));

  echo $name.$pad.$num."\n";
    ob_start();
  return gethrtime();
}

function total()
{
  global $total;
  $pad = str_repeat("-", 24);
  echo $pad."\n";
  $num = number_format($total,3);
  $pad = str_repeat(" ", 24-strlen("Total")-strlen($num));
  echo "Total".$pad.$num."\n";
}

$t0 = $t = start_test();
simple();
$t = end_test($t, "simple");
simplecall();
$t = end_test($t, "simplecall");
simpleucall();
$t = end_test($t, "simpleucall");
simpleudcall();
$t = end_test($t, "simpleudcall");
mandel();
$t = end_test($t, "mandel");
mandel2();
$t = end_test($t, "mandel2");
ackermann(7);
$t = end_test($t, "ackermann(7)");
ary(50000);
$t = end_test($t, "ary(50000)");
ary2(50000);
$t = end_test($t, "ary2(50000)");
ary3(2000);
$t = end_test($t, "ary3(2000)");
fibo(30);
$t = end_test($t, "fibo(30)");
hash1(50000);
$t = end_test($t, "hash1(50000)");
hash2(500);
$t = end_test($t, "hash2(500)");
heapsort(20000);
$t = end_test($t, "heapsort(20000)");
matrix(20);
$t = end_test($t, "matrix(20)");
nestedloop(12);
$t = end_test($t, "nestedloop(12)");
sieve(30);
$t = end_test($t, "sieve(30)");
strcat(200000);
$t = end_test($t, "strcat(200000)");
total();
?>

Results with JIT disabled

Run the benchmark with JIT turned off by setting the JIT buffer size to zero:

php -d opcache.jit_buffer_size=0 Zend/bench.php

Output:

simple 0.008

simplecall 0.004

simpleucall 0.004

simpleudcall 0.004

mandel 0.035

mandel2 0.055

ackermann(7) 0.020

ary(50000) 0.004

ary2(50000) 0.003

ary3(2000) 0.048

fibo(30) 0.084

hash1(50000) 0.013

hash2(500) 0.010

heapsort(20000) 0.027

matrix(20) 0.026

nestedloop(12) 0.023

sieve(30) 0.013

strcat(200000) 0.006

------------------------

Total 0.387

Results with JIT enabled

Since bench.php is a script-style workload, opcache.jit=1205 is the appropriate choice here:

php -d opcache.jit_buffer_size=64M -d opcache.jit=1205 Zend/bench.php